The present invention relates to energy transmission lines. In particular, the present invention relates to a shielded, diaxial transmission line that is well-suited to the transmission of electrical power used in an advantageous apparatus and method for using a continuous conductor, such as oil well piping, as a dipole antenna to transmit radio frequency (“RF”) energy for heating.
As the world's standard crude oil reserves are depleted, and the continued demand for oil causes oil prices to rise, oil producers are attempting to process hydrocarbons from bituminous ore, oil sands, tar sands, and heavy oil deposits. These materials are often found in naturally occurring mixtures of sand or clay. Because of the extremely high viscosity of bituminous ore, oil sands, oil shale, tar sands, and heavy oil, the drilling and refinement methods used in extracting standard crude oil are typically not available. Therefore, recovery of oil from these deposits requires heating to separate hydrocarbons from other geologic materials and to maintain hydrocarbons at temperatures at which they will flow. Steam is typically used to provide this heat in what is known as a steam assisted gravity drainage system, or SAGD system. Electric and RF heating are sometimes employed as well. The heating and processing can take place in-situ, or in another location after strip mining the deposits.
Heating subsurface heavy oil bearing formations by prior RF systems has been inefficient due to traditional methods of matching the impedances of the power source (transmitter) and the heterogeneous material being heated, uneven heating resulting in unacceptable thermal gradients in heated material, inefficient spacing of electrodes/antennae, poor electrical coupling to the heated material, limited penetration of material to be heated by energy emitted by prior antennae and frequency of emissions due to antenna forms and frequencies used. Antennas used for prior RF heating of heavy oil in subsurface formations have typically been dipole antennas. U.S. Pat. Nos. 4,140,179 and 4,508,168 disclose prior dipole antennas positioned within subsurface heavy oil deposits to heat those deposits.
Arrays of dipole antennas have been used to heat subsurface formations. U.S. Pat. No. 4,196,329 discloses an array of dipole antennas that are driven out of phase to heat a subsurface formation.
Magnetic and electric fields are frequently produced at the power transmission lines of dipole antennas. In general, the overburden in a subsurface formation is more conductive than the ore in general. Thus, the application of electric and magnetic fields to the overburden through power transmission lines used for RF heating may be conducted preferentially to the overburden rather than the target formation.